1. Field of the Invention
The present invention relates to magnetic fluids and a process for preparing the same. Particularly, the present invention relates to magnetic fluids using a silane-based surface modifier, which is a nondispersant, as a surfactant-accepting layer on the magnetic particles before applying a surfactant. More particularly, the present invention relates to magnetic fluids using a silane-based surface modifier, which is a nondispersant, as a surfactant-accepting layer on the magnetic particles which improves chemical stability for silicone-based, hydrocarbon-based and ester-based magnetic fluids. Yet more particularly, the present invention relates to magnetic fluids using a silane-based surface modifier which allows the use of surfactants that were not previously useable as first surfactants in oil-based magnetic fluids due to their chemical nature. Yet even more particularly, the present invention relates to a process for making stable magnetic fluids having a silane-based surface modifier, which is a nondispersant, as a surfactant-accepting layer.
2. Description of the Prior Art
Magnetic fluids, sometimes referred to as "ferrofluids" or magnetic colloids, are colloidal dispersions or suspensions of finely divided magnetic or magnetizable particles ranging in size between thirty and one hundred fifty angstroms and dispersed in a carrier liquid. One of the important characteristics of magnetic fluids is their ability to be positioned and held in space by a magnetic field without the need for a container. This unique property of magnetic fluids has led to their use for a variety of applications. One such use is their use as liquid seals with low drag torque where the seals do not generate particles during operation as do conventional seals. These liquid seals are widely used in computer disc drives as exclusion seals to prevent the passage of airborne particles or gases from one side of the seal to the other. In the environmental area, environmental seals are used to prevent fugitive emissions, that is emissions of solids, liquids or gases into the atmosphere, that are harmful or potentially harmful.
Other uses of magnetic fluids are as heat transfer fluids between the voice coils and the magnets of audio speakers, as damping fluids in damping applications and as bearing lubricants in hydrodynamic bearing applications. Yet another is their use as pressure seals in devices having multiple liquid seals or stages such as a vacuum rotary feedthrough seal. Typically, this type of seal is intended to maintain a pressure differential from one side of the seal to the other while permitting a rotating shaft to project into an environment in which a pressure differential exists.
The magnetic particles are generally fine particles of ferrite prepared by pulverization, precipitation, vapor deposition or other similar means. From the viewpoint of purity, particle size control and productivity, precipitation is usually the preferred means for preparing the ferrite particles. The majority of industrial applications using magnetic fluids incorporate iron oxides as magnetic particles. The most suitable iron oxides for magnetic fluid applications are ferrites such as magnetite or .gamma.-ferric oxide, which is called maghemite. Ferrites and ferric oxides offer a number of physical and chemical properties to the magnetic fluid, the most important of these being saturation magnetization, viscosity, magnetic stability, and chemical stability of the whole system. To remain in suspension, the ferrite particles require a surfactant coating, also known as a dispersant to those skilled in the art, in order to prevent the particles from coagulating or agglomerating.
Fatty acids, such as oleic acid, have been used as dispersing agents to stabilize magnetic particle suspensions in some low molecular-weight non-polar hydrocarbon liquids. These low molecular-weight non-polar hydrocarbon liquids are relatively volatile solvents such as kerosene, toluene and the like. Due to their relative volatility, evaporation of these volatile hydrocarbon liquids is an important drawback as it deteriorates the function of the magnetic fluid itself. Thus to be useful, a magnetic fluid must be made with a low vapor-pressure carrier liquid and not with a low-boiling point hydrocarbon liquid. However, the hydrocarbon-based ferrofluids have been limited in some applications because of a relatively large change in viscosity as a function of temperature.
The surfactants/dispersants have two major functions. The first is to assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic attraction, i.e. to prevent coagulation or agglomeration. The second is to provide a chemical composition on the outer surface of the magnetic particle that is compatible with the liquid carrier.
The saturation magnetization (G) of magnetic fluids is a function of the disperse phase volume of magnetic materials in the magnetic fluid. In magnetic fluids, the actual disperse phase volume is equal to the phase volume of magnetic particles plus the phase volume of the attached dispersant. The higher the magnetic particle content, the higher the saturation magnetization. The type of magnetic particles in the fluid also determines the saturation magnetization of the fluid. A set volume percent of metal particles in the fluid such as cobalt and iron generates a higher saturation magnetization than the same volume percent of ferrite. The ideal saturation magnetization for a magnetic fluid is determined by the application. For instance, saturation magnetization values for exclusion seals used in hard disk drives are typically lower than those values for vacuum seals used in the semiconductor industry.
Most of the magnetic fluids employed today have one to three types of surfactants arranged in one, two or three layers around the magnetic particles. The surfactants for magnetic fluids are long enough chain and a functional group at one end. The chain may also contain aromatic hydrocarbons. The functional group can be cationic, anionic or nonionic in nature. The functional group is attached to the outer layer of the magnetic particles by either chemical bonding or physical force or a combination of both. The chain or tail of the surfactant provides a permanent distance between the particles and compatibility with the liquid carrier.
Various magnetic fluids and the processes for making the same have been devised in the past. The oil-based carrier liquid is generally an organic molecule, either polar or nonpolar, of various chemical compositions such as hydrocarbon (polyalpha olefins, aromatic chain structure molecules), esters (polyol esters), silicone, or fluorinated and other exotic molecules with a molecular weight range up to about eight to nine thousand. Most processes use a low boiling-point hydrocarbon solvent to peptize the ferrite particles. To evaporate the hydrocarbon solvent from the resultant oil-based magnetic fluid in these processes, all of these processes require heat treatment of the magnetic fluid at about 70.degree. C. and higher or at a lower temperature under reduced pressure. Because there are a number of factors that affect the physical and chemical properties of the magnetic fluids and that improvements in one property may adversely affect another property, it is difficult to predict the effect a change in the composition or the process will have on the overall usefulness of a magnetic fluid. It is known in the art that magnetic fluids in which one of the dispersants is a fatty acid, such as oleic, linoleic, linolenic, stearic or isostearic acid, are susceptible to oxidative degradation of the dispersant system. This results in gelation of the magnetic fluid.
Silicone oils have been suggested as liquid carriers in ferrofluid compositions and for use in loudspeakers. However, stable silicone oil-based ferrofluids have been difficult to synthesize in practice. Past attempts to synthesize silicone oil-based ferrofluids, utilizing such surfactants as oleic acid, have had a very limited success. With oleic-acid-type surfactants, only ferrofluids based on silicones having very low molecular weights have been prepared with undesirable high evaporation rates of the silicone. In addition, the use of other surfactants also has proven to be unsatisfactory in preparing silicone-based ferrofluids, since such silicone-based fluids have not proven to be stable in a magnetic or gravity field, either during storage or during use.
The surfactant, which keeps the ferrofluid particles dispersed, is critical in proper ferrofluid operation. Ferrofluids with multiple surfactants have been conventionally used. One such ferrofluid is described in U.S. Pat. No. 4,956,113.
U.S. Pat. No. 4,956,113 (1990, Kanno et al.) teaches a process for preparing a magnetic fluid. The magnetic fluid contains fine particles of ferrite stably dispersed in low vapor pressure base oil. The magnetic fluid is prepared by adding N-polyalkylenepolyamine-substituted alkenylsuccinimide to a suspension of fine particles of surfactant-adsorbed ferrite dispersed in a low boiling point hydrocarbon solvent. The surfactant adsorbed on the fine particles of ferrite is one of those usually used for dispersing fine particles into a hydrocarbon solvent, preferably higher fatty acid salts and sorbitan esters. The mixture is heated to remove the hydrocarbon solvent followed by the addition of low vapor pressure base oil and a specific dispersing agent. The resultant mixture is subjected to a dispersion treatment.
It is known that ferrofluids can be prepared using a wide variety of liquid carriers including hydrocarbons, such as kerosene or heptane, aromatics such as toluene, xylene or styrene, and diesters such as ethylhexyl azelate, as well as other aqueous solutions, alcohols, acetates or ethers. However, present day hydrocarbon and ester based ferrofluids have been limited in some applications because the liquid carrier generally exhibits a relatively large change in viscosity as a function of temperature. Silicone oils (polysiloxanes) can be used as liquid carriers in ferrofluid compositions. In particular, high molecular weight polydimethylsiloxane (PDMS) oils exhibit a relatively small change in viscosity and possess a wide thermal range of operation. Therefore, ferrofluids made with PDMS oils can be used in environments where hydrocarbon and ester based ferrofluids are not readily suited.
Long-term stable and concentrated silicone oil-based ferrofluids have been difficult to synthesize in practice due, in part, to the unavailability of a satisfactory surfactant system. U.S. Pat. No. 4,356,098 (1982, Chagnon) discloses a ferrofluid with a silicone oil carrier which uses a single silicone oil surfactant. However, it has been found that the single silicone oil surfactant attaches poorly to the surface of the magnetic particles. In addition, the silicone-based ferrofluid tends to polymerize and congeal in a short period of time so that it loses its original fluid properties.
U.S. Pat. No. 5,851,416 (1998, Raj et al.) discloses silicone oil-based ferrofluid comprising a colloidal dispersion of finely divided magnetic particles in a silicone oil carrier. The surfaces of the magnetic particles are modified with a first surfactant comprising a hydrocarbon having at least one polar group and a second surfactant comprising a silicone oil surfactant having at least one polar group and which is soluble in the silicone oil carrier. It is believed that a ferrofluid based on this disclosure has a poor gel time mostly because of the large hydrocarbon tail provided by the oleic acid. It is well known that a large hydrocarbon molecule cannot dissolve in a silicone and that a large hydrocarbon molecule makes the whole system unstable. It is also believed that use of a large amount of surfactant with a carrier oil having relatively high viscosity contributes to a relatively low maximum saturation magnetization and high viscosity of the product.
All of the prior art uses one, two or three surfactants to disperse the magnetic particles in a carrier liquid. There is further a limited selection for a first dispersant that is capable of being adsorbed on magnetic particles and disperse them in carrier liquid. There is also prior art that suggests the use of a low molecular weight surface modifier as an additive to a ferrofluid.
U.S. Pat. No. 5,676,877 (1997, Borduz et al.) discloses a ferrofluid composition and a process for producing a chemically stable magnetic fluid comprising finely divided magnetic particles covered with surfactants. A surface modifier is also employed, which is added after adsorption of the surfactants, to cover thoroughly the free oxidizable exterior surface of the outer layer of the particles not covered by the surfactants.
None of the prior art proposes or suggests the use of low molecular weight surface modifiers, which are nondispersants, as surface modifiers to cover the surface area of the magnetic particles prior to adsorption of larger-sized surfactants.
Therefore, what is needed is a magnetic fluid that has a low molecular weight surface modifier covering the surface area of the magnetic particles before attachment of larger-sized surfactants. What is also needed is a magnetic fluid that has a low molecular weight silane-based surface modifier covering the surface area of the magnetic particles before attachment of larger-sized surfactants. What is further needed is a magnetic fluid that has a low molecular weight alkyl alkoxy silane based surface modifier covering the surface area of the magnetic particles before attachment of larger-sized surfactants. What is yet further needed is a silicone oil-based, hydrocarbon-based or ester-based magnetic fluid that has a low molecular weight alkyl alkoxy silane based surface modifier covering the surface area of the magnetic particles, which allows the use of surfactants that were not previously useable as first surfactants or that required a complicated process to be useable as a first surfactant. Finally what is needed is a process for making a silicone oil-based, hydrocarbon oil-based and ester oil-based magnetic fluid that has increased chemical stability.